1. Field of the Invention
The present invention generally relates to an image processor, and more particularly, to an encoder and a decoder for HD Photo.
2. Description of the Background Art
Microsoft Corporation proposes HD Photo (or JPEG XR) as a still image file format that offers higher image quality than JPEG while requiring more simple circuit configuration and computation than JPEG 2000.
An encoder for HD Photo includes a frequency transform unit performing predetermined frequency transform (PCT), and a pre-filter performing predetermined prefiltering so as to reduce block artifacts. The frequency transform unit performs frequency transform with a pixel block having 4 pixels in column×4 pixels in row as a unit region for processing. The pre-filter performs prefiltering with a region which overlaps with plural unit regions for processing by the frequency transform unit as a unit region for processing, before frequency transform is performed.
A decoder for HD Photo includes a frequency inverse transform unit performing frequency inverse transform that corresponds to the above frequency transform, and a post-filter performing postfiltering that corresponds to the above prefiltering. The frequency inverse transform unit performs frequency inverse transform with a pixel block having 4 pixels in column×4 pixels in row as a unit region for processing. The post-filter performs postfiltering with a region which overlaps with plural unit regions for processing by the frequency inverse transform unit as a unit region for processing, after frequency inverse transform is performed.
HD Photo provides supports for images represented by RGB color space. In addition, HD Photo supports images represented by YUV color space. For processing an image of YUV color space, the encoder for HD Photo includes a luminance signal processing unit for processing a luminance signal (Y signal), and a chrominance signal processing unit for processing a chrominance signal (U and V signals). The luminance signal processing unit and the chrominance signal processing unit each includes the above pre-filter and frequency transform unit. Alternatively, the luminance signal processing unit and the chrominance signal processing unit share one pre-filter and one frequency transform unit. Similarly, for processing an image of YUV color space, the decoder for HD Photo includes a luminance signal processing unit for processing a luminance signal, and a chrominance signal processing unit for processing a chrominance signal. The luminance signal processing unit and the chrominance signal processing unit each includes the above post-filter and frequency inverse transform unit. Alternatively, the luminance signal processing unit and the chrominance signal processing unit share one post-filter and one frequency inverse transform unit.
The details of HD Photo are disclosed in, for example, “HD Photo—Photographic Still Image File Format”, [online], 7 Nov. 2006, Microsoft Corporation, [searched in the Internet on 10 Oct. 2007], <URL: http://www.microsoft.com/whdc/xps/hdphotodpk.mspx>. The details of JPEG XR are disclosed in, for example, “Coding of Still Pictures -JBIG JPEG”, [online], 19 Dec. 2007, ISO/IEC JTC 1/SC 29/WG1 N 4392, [searched in the Internet on 4 Mar. 2008], <URL: http://www.itscj.ipsj.or.jp/sc29/open/29view/29n9026t.doc>.
In an image of YUV444 format, a pixel plane has the same number of Y, U, and V signals within one row. Thus the luminance signal processing unit and the chrominance signal processing unit in the encoder and the decoder for HD Photo can process each of the Y, U, and V signals with respect to a pixel in the same position within the pixel plane.
In contrast, in an image of YUV422 Format, a pixel plane has half number of U and V signals each of the number of Y signals within one row. Thus in the encoder and the decoder for HD Photo, some measures need to be taken to prevent difference in the position within a pixel plane between a pixel processed by the luminance signal processing unit and a pixel processed by the chrominance signal processing unit.